A magnetic disk drive includes circular data tracks on a rotating magnetic disk and read and write heads that may form a merged head attached to a slider on a positioning arm. During a read or write operation, the merged head is suspended over the magnetic disk on an air bearing surface (ABS). The sensor in a read head is a critical component since it is used to detect magnetic field signals by a resistance change. One form of magnetoresistance is a spin valve magnetoresistance (SVMR) or giant magnetoresistance (GMR) which is based on a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) or pinning layer. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. The rotation of magnetization in the free layer relative to the fixed layer magnetization generates a resistance change that is detected as a voltage change when a sense current is passed through the structure. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head. To meet this requirement, the CPP configuration is a stronger candidate than the CIP configuration which has been used in recent hard disk drives (HDDs). The CPP configuration is more desirable for ultra-high density applications because when the power consumption in the sensor is made constant to avoid a temperature rise, the output voltage is roughly inversely proportional to the square root of the sensor area. Therefore, a stronger output signal is achieved as the sensor size decreases. The sensor area at the ABS plane for greater than 100 Gb/in2 density is smaller than 0.1×0.1 microns.
An important characteristic of a GMR head is the magnetoresistive (MR) ratio which is dR/R where dR is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A higher ratio is desired for improved sensitivity in the device and this result is achieved when electrons in the sense current spend more time within the magnetically active layers of the sensor. Bulk scattering in thicker magnetic layers improves the MR ratio but has limitations because a larger sensor size will affect the output voltage. Interfacial scattering which is the specular reflection of electrons at the interfaces between layers in the sensor stack also improves the MR ratio and thereby increases sensitivity.
In a CPP operation mode, a tunnel magnetoresistive (TMR) head as described by K. Ohashi et al. in “The Applicability of CPP-GMR Heads for Magnetic Recording”, IEEE Trans. on Magnetism, Vol. 38, pp. 2277-2282 (2002), is a candidate for realizing high sensitivity. However, the TMR head has several disadvantages of which one is a large resistance that limits the operating frequency and makes the Johnson and shot noise high. It is considered too difficult to reduce TMR head resistance without a breakthrough in fabrication technology for low resistance barrier layers. For example, the current aluminum oxide barrier layer made from two atomic (111) Al layers is not pinhole free and thus has a very low dielectric breakdown voltage.
A CPP-GMR head is preferred over a TMR head design for ultra-high density recording because the former has a lower impedance. However, the resistance (RA) in a conventional single spin valve is too small (<100 mohm-μm2) and the MR ratio of a CPP head is very low (<0.5%). Additionally, the output voltage which is related to the resistance change is too low. One way to increase the resistance change is to optimize the materials and structure of the CPP-GMR head.
In a CIP spin valve, the resistance change dR is related to the output amplitude. For a CPP spin valve, the corresponding resistance term is RA and the resistance change (output signal) is dRA. In a CPP operation mode, the MR ratio in the active area is determined by the materials in the pinned, spacer, and free layers and is related to the spin polarization at the interfaces between the pinned layer and spacer layer and between the spacer layer and free layer. Consequently, the resistance RA can be increased by implementing a thicker magnetic layer or with multi-magnetic layers. The former increases bulk scattering and the latter increases interfacial scattering. A very thin non-magnetic layer (dusting) can be inserted into a magnetic layer as an alternative method to increase interfacial scattering and RA.
A nano-oxide layer such as FeTaO described in U.S. Pat. No. 6,581,272 is used to increase specular reflection and improve the MR ratio in a bottom spin valve sensor having a synthetic anti-parallel pinned layer. A FeTaO layer about 5 Angstroms thick is inserted within a second CoFe pinned layer that is separated from a first CoFe pinned layer by a Ru coupling layer.
A synthetic anti-parallel (SyAP) spin valve sensor has a pinned layer structure that is a composite of multiple layers. Typically, a coupling layer is sandwiched between a first pinned layer and a second pinned layer that have slightly different thicknesses. The first pinned layer has a magnetic moment or vector oriented in a first direction by exchange coupling with an AFM pinning layer. The second pinned layer is adjacent to the free layer and is antiparallel exchange coupled to the first pinned layer via the coupling layer. Thus, the magnetic moment or vector of the second pinned layer is oriented in a second direction that is anti-parallel to the magnetic vector of the first pinned layer. The magnetic moments of the first and second pinned layers combine to produce a net magnetic moment that is less than the magnetic moment of a single pinned layer. A small net magnetic moment results in improved exchange coupling between the first pinned layer and the AFM layer and also reduces interlayer coupling between the pinned layer and the free layer.
As found in CIP heads, a higher MR ratio is realized by a dual type spin valve design in which two spin valves are formed adjacent to one another in the same sensor stack. Similarly, in a CPP head, a dual spin valve structure is preferred for improved performance. Thicker magnetic layers may be implemented to increase bulk scattering. It is noted that interfacial scattering is doubled in a dual CPP head compared to a single spin valve structure since the former has four magnetic/non-magnetic interfaces. Thus, RA and dRA which is the product of RA and the MR ratio can be greatly improved in the dual spin valve structure.
A sputtering target containing Mn, one other metal, and less than 1% oxygen is employed to form an AFM layer in a magnetic sensor in a read head as disclosed in U.S. Pat. No. 6,165,607. By incorporating oxygen in the sputtering target and presumably in the AFM layer, a more stable and homogeneous AFM layer is produced that leads to a higher exchange bias force.
A small amount of oxygen doped into an AFM layer has been used to improve the exchange bias field as reported by H. Fuke et al. in “Influence of Crystal Structure and Oxygen Content on Exchange-Coupling Properties of IrMn/CoFe SV Films” in Applied Phys. Letters, Vol. 75, pp. 3680-3682 (1999).
An AFM layer containing oxygen is also disclosed in U.S. Pat. No. 6,331,773 where a NiO AFM layer is used in a bottom spin valve structure to provide good wear properties. However, a NiFe oxidation protection layer that does not disrupt exchange bias is required between the NiO AFM layer and a synthetic pinned layer.
In U.S. Patent Application Publication No. 2004/0004261, a GMR element is described in which free and pinned layers have conduction electrons of different energy bands and are comprised of a half metal such as Fe3O4 which may be formed by molecular beam epitaxy with O2 being admitted into the atmosphere while Fe is being deposited.
In U.S. Pat. No. 6,621,666, hard magnetic (bias) layers comprised of CoFe2O3 are formed on opposite sides of a spin valve stack but are not in the MTJ cell itself. The CoFe2O3 deposition involves a CoFe alloy target and Ar-10% O2 as a sputtering gas.
An Al2O3 or NiCrOx insulating barrier is disclosed in U.S. Pat. No. 6,574,079 and is formed at low O2 pressure during oxidation of an Al or NiCr layer.
In U.S. Patent Application Publication No. 2003/0184918, a longitudinal bias stack is formed between two spin valves and provides flux closures for the sense layers in the spin valve stacks. Copper oxide spacers are formed by magnetron sputtering with an Ar and O2 gas mixture to reduce ferromagnetic coupling between pinned and sense layers.
A ferromagnetic nanocomposite layer comprised of CoFe nanogranules and an HfO intergranular matrix is described in U.S. Patent Application Publication No. 2002/0146580. A sputter deposition process involves a CoFeHf target and a trace amount of O2 in the Ar sputter gas.